[B-PT04] 地球生命史

[BPT04-P04] 北海道日高町波恵川における平取断層付近の中新統放散虫・珪藻化石層序

*本山 功¹、庄司 絵利加²、秋葉 文雄³ (1.山形大学理学部、2.山形大学大学院理工学研究科、3.（有）珪藻ミニラボ)

キーワード：放散虫、珪藻、滝の上動物群

According to previous studies, the Middle Miocene Abetsu and Nibutani Formations and the Upper Miocene Nina Formation are in contact with each other across the Biratori Fault in the vicinity of the Hae River, Hidaka Town, central Hokkaido. However, no exposure of the fault has been observed so far in this area, and it is difficult to classify strata on the basis of lithofacies alone, since the strata in this area are mainly composed of fairly monotonous mudstone although accompanied by minor sandstone and conglomerate. Therefore, we determined the age of the strata and fault locations, taking radiolarian and diatom stratigraphy into consideration. As a result, the radiolarian Lithocampe subligata, Dendrospyris sakaii, lower Eucyrtidium inflatum and Lychnocanoma magnacornuta Zones and the diatom NPD3B, NPD4A, NPD4B and NPD5C Zones were recognized, with the lack of the subzone B of the E. inflatum Zone and the NPD5A and NPD5B Zones approximating the location of the fault. By digging the gravel of the riverbed at the presumed location and observing the bedrock, we were able to confirm the fault outcrop where the Abetsu and Nina Formations are in contact with each other across the fault crush zone. The estimated age of the Sakae Formation indicates that the Takinoue Fauna is correlated with the Burdigalian stage.